"I think, with a lot of dramas, people come back to them hoping to see the same characters, and be assured by the same characters every week. You know what you're getting most of the time when you tune into a drama. What we always hoped with Black Mirror is that you don't know what you're getting..."

So said writer and series creator Charlie Brooker in a recent BAFTA Q&A, which took place after a private screening of series two's opening episode, Be Right Back. Of all the Black Mirror episodes we've seen so far, White Bear fulfilled that remit outlined above; its horror-infused drama leaves us unsure whom we can trust or what will happen next, and its last act is truly gut-wrenching.

Like protagonist Victoria (Lenora Crichlow), we're left bewildered by the episode's assaultive opening. Waking up in a chair to the hiss of TV static, Victoria finds herself in an apparently ordinary house. Her wrists have been bandaged, possibly from a self-inflicted wound; a little sprinkling of pills lies at her feet.

Confused, Victoria ventures outside, emerging into the autumn gloom of a British housing estate. Its residents stand eerily silent behind plastic double glazing, recording Victoria's every movement on their mobile phones. We're barely given more than a few seconds to ponder the meaning of all this before a blue car pulls up, a masked man exits, withdraws a shotgun from the boot, and takes careful aim at Victoria's head.

Just as Be Right Back appeared to mark a growing confidence in Charlie Brooker's writing, with its subtle approach giving the events and performances greater impact, so White Bear sees Brooker pare back his use of dialogue to an extent rarely seen in television drama. These opening scenes, which are almost bereft of chatter other than Victoria's plaintive "Can you help me? I can't remember who I am," are all the more effective for their economy, and the subsequent chase, which begins with that masked figure pulling a gun from his car, is truly the stuff of nightmares - like Kafka with added mobile phone cameras.

Admittedly, the use of amnesia isn't the most original of plot devices, but the fearsome pace director Carl Tibbetts brings to the story's first act gives us little time to reflect on this. Before we know it, Victoria's survived a Dawn Of The Dead-like encounter at a petrol station, and teamed up with Jem (Tuppence Middleton), a feisty young woman who has a much stronger handle on the situation. The population, she tells us, has been enslaved by subliminal messages on TVs and mobile phones. While most have become zombie-like voyeurs, one in ten have devolved into hunters, like the one with the mask and the shotgun. "They seemed normal," Jem explains, "but then they realised they could do what they like. And now they've got an audience."

As a helpful rescuer (played by Kill List's Michael Smiley) turns out to be another vicious hunter, the sense of horror keeps building; the use of woodland settings, crucified bodies and the gleeful waving around of power tools evokes the memory of a legion 70s exploitation movies, helped along by Crichlow's uncanny ability to emit blood-curdling screams.

If there's a criticism to be levelled at the first two-thirds of White Bear, it's that Victoria's carried helplessly along by events. It's only at the end of the second act that Victoria stops crying and finally takes control, and pulls the trigger on a shotgun herself - and that sputtering blast signals the plot's icily abrupt left turn.

As what turns out to be a television set parts to reveal a baying studio audience, it briefly seems as though Victoria has been involved in some sort of Derren Brown-style hypnotic experiment - like last year's Apocalypse, perhaps, where a lazy young man was prodded out of inactivity by being fooled into thinking he was trapped in the middle of a zombie armageddon.

Instead, the truth is revealed to be much, much darker: Victoria is a criminal, convicted for her part in the brutal killing of a young girl. As punishment, she's subjected to the repeated physical and psychological torture of Whitebear Justice Park, an enclosed space where the public can observe and record her daily suffering, like a cross between Michael Crichton's West World and Woburn Safari Park.

The final act is powerful for several reasons. While the rug-pull turn of events is a great one for those who didn't expect it, the truth of Victoria's past is more shocking still. The similarities between real-life murderers - Ian Huntley and Maxine Carr, Ian Brady and Myra Hindley - are obvious, the prolonged scenes of Michael Smiley's master tormentor and his enraged crowd disquieting; there are several visual and thematic parallels to The Wicker Man and Kill List, in which Smiley also starred.

Nasty though White Bear is, with its allusions to real-life witch hunts often led by red-top newspapers, it's more than mere attention-seeking: there's a certain sense of morality underlying this episode, as there so often is in Black Mirror. White Bear explores how human empathy breaks down when individuals are reduced to an image on a screen, and concludes, quite rightly, that whether it's directed at the innocent or the guilty, cruelty is still cruelty.

Once again, strong ideas, intelligent writing and great direction came together to create a disturbing reflection of the present.